I have tested this at the demo site and confirmed it. If you enable WYSIWYG editor (tinyMCE) on comments and just enter the URL for an image; the image does show in the editor, and click the post button it returns Please enter 3-2048 characters. To tell you the truth, I don't care if someone leaves a blanc post, doesn't bother me in the least. Why the 3 by the way, what if I wanted to just say "Hi" on someone's comments; how about 2-2048 or even 1 if I just want to say "I" which is a word, LOL.
However, leaving just an image was not a problem with 7.1; I know because I have an active 7.1 site where this happens all the time. So, this must have been introduced with 7.1.2, either on purpose or by accident. If on purpose, it needs to be changed as leaving just an image in a comment is valid and is done all the time.
